Highly efficient separation of methane from nitrogen on a squarate‐based metal‐organic framework

Highly selective capture of methane from nitrogen is considered to be a feasible approach to improve the heating value of methane and mitigate the effects of global warming. In this work, an ultramicroporous squarate‐based metal‐organic framework (MOF), [Co₃(C₄O₄)₂(OH)₂] (C₄O₄^2? = squarate), with enhanced negative oxygen binding sites was synthesized for the first time and used as adsorbent for efficient separation of methane and nitrogen. Adsorption performance of this material was evaluated by single‐component adsorption isotherms and breakthrough experiments. Furthermore, density functional theory calculation was performed to gain the deep insight into the adsorption binding sites. Compared with the other state‐of‐the‐art materials, this material exhibited the highest adsorption selectivity (8.5–12.5) of methane over nitrogen as well as the moderate volumetric uptake of methane (19.81 cm³/cm³) under ambient condition. The unprecedented selectivity and chemical stability guaranteed this MOF as a candidate adsorbent to capture CH₄ from N₂, especially for the unconventional natural gas upgrading. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3681–3689, 2018     

High‐throughput and comprehensive prediction of H2 adsorption in metal‐organic frameworks under various conditions

High‐throughput prediction of H₂ adsorption in metal‐organic framework (MOF) materials has been extended from a few specific conditions to the whole T, p space. The prediction is based on a classical density functional theory and has been implemented over 712 MOFs in 441 different conditions covering a wide range. Some testing materials show excellent behavior at low temperatures and obvious improvement at high temperatures compared to conventional MOFs. The structures of the best MOFs at high and low temperatures are totally different. Linear and nonlinear correlations between the two Langmuir parameters have been found at high and low temperatures, respectively. According to the analysis of the excess uptake, we found that the saturated pressure increases along with temperature in the low temperature region but decreases in the high temperature region. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2951–2957, 2015     

气体在金属-有机骨架材料中的吸附分离:经典密度泛函理论的应用

回顾了近年来经典密度泛函理论在预测金属-有机骨架材料吸附特性方面的研究进展,重点介绍了经过快速傅里叶变换加速后的经典密度泛函理论在MOF吸附材料的大规模筛选方面的应用。相较传统的计算机分子模拟,加速后的经典密度泛函理论的优势在于计算效率,对于简单的小分子气体系统尤其具有优势,对MOF吸附材料进行大规模筛选是可行的;但对于对复杂分子的处理尚没有特别有效的方法,如何合理构建复杂流体自由能泛函是它面临的主要挑战。     

Water adsorption in metal–organic frameworks with open‐metal sites

H₂O adsorptions inside porous materials, including silica zeolites, zeolite imidazolate frameworks, and metal–organic frameworks (MOFs) using molecular simulations with different water models are investigated. Due to the existence of coordinately unsaturated metal sites, the predicted adsorption properties in M‐MOF‐74 (M = Mg, Ni, Co, Zn) and Cu‐BTC are found to be greatly sensitive to the adopted H₂O models. Surprisingly, the analysis of the orientations of H₂O minimum energy configuration in these materials show that three‐site H₂O models predict an unusual perpendicular angle of H₂O plane with respect to the Metal‐O₄ plane, whereas those models with more than three sites give a more parallel angle that is in better agreement with the one obtained from density functional theory (DFT) calculations. In addition, the use of these commonly used models estimates the binding energies with the values lower than the ones computed by DFT ranging from 15 to 40%. To correct adsorption energies, simple approach to adjust metal‐O(H₂O) sigma parameters to reproduce the DFT‐calculated binding energies is used. With the refined parameters, the computed water isotherms inside Mg‐MOF‐74 and Cu‐BTC are in reasonable agreement with experimental data, and provide significant improvement compared to the predictions made by the original models. Further, a detailed inspection on the water configurations at higher‐pressure region was also made, and observed that there is an interesting two‐layer water network formed using three‐ and four‐site models. © 2014 American Institute of Chemical Engineers AIChE J, 61: 677–687, 2015     

Efficient adsorption separation of acetylene and ethylene via supported ionic liquid on metal‐organic framework

Ionic liquid (IL) supported metal‐organic framework (MOF) was utilized to efficiently separate acetylene from ethylene. A common IL, 1‐butyl‐3‐methylimidazolium acetate ([Bmim][OAc]), was encapsulated into a hydrothermally stable MOF, namely MIL‐101(Cr). Characterization techniques including FTIR, Powder X‐ray diffraction, BET, and thermal gravimetric analysis were used to confirm successful encapsulation of the IL within MIL‐101(Cr). Adsorption isotherms of acetylene and ethylene in the IL‐encapsulated MOF were tested. From the results, the MOF composite retained a relatively high adsorption capacity. Remarkably, the adsorption selectivity of acetylene/ethylene has dramatically increased from 3.0 to 30 in comparison with the parent MIL‐101(Cr). Furthermore, the potential of industrial practice was examined by breakthrough and regeneration experiments. It not only satisfies the industrial production of removal of low level of acetylene from ethylene, but also is notably stable during the adsorption‐desorption process. The high designability of ILs combined with richness of MOFs’ structures exploits a novel blueprint for gas separation. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2165–2175, 2017     

Development of dual-model classical density functional theory and its application to gas adsorption in porous materials

Classical density functional theory (CDFT) is a useful theory in many fields. The basis of CDFT is spherical model and extending it to nonspherical molecules is a challenging issue due to the orientation/configuration of the molecules, which implies more complicated molecular models, and higher computational costs. In this work, we propose a dual-model classical density functional theory (DM-CDFT) to address this issue. The theory uses a more precise model (all-atom model) and a simpler model (coarsening model) to construct the external and excess free energy functionals, respectively. By using this methodology, CDFT could handle orientation/configuration effects with low computational costs. The theory is examined by applying it to gas adsorption (such as C₂H₂/C₂H₄/C₂H₆ and toxic gases) in porous materials, and the predicted adsorption isotherms verify the accuracy of the theory. Additionally, the predicted density profile indicates that rotation entropy plays an important role in the adsorption of nonspherical molecules.     

Fabrication of homochiral metal‐organic framework membrane for enantioseparation of racemic diols

Chiral metal‐organic frameworks (MOFs) used to discriminate chiral enantiomers are of great practical significance. In this study, a novel homochiral [Ni₂(L‐asp)₂(bipy)] membrane was fabricated on a porous ceramic support and used for enantioselective separation of racemic diols. High‐energy ball milling was applied to decrease the size of MOF crystals to achieve homogeneous seed suspensions. A high‐quality homochiral membrane was obtained after optimizing the preparation process. Under the concentration‐driven permeation process, racemic 2‐methyl‐2,4‐pentanediol (MPD) was readily separated by the as‐prepared membrane. At 30°C, an enantiomeric excess value of 35.5 ± 2.5% was obtained at a feed concentration of 1.0 mmol L^?1. The chiral separation of racemic MPD via the membrane followed a preferential sorption mechanism. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4364–4372, 2013     

Catalytic dehydration of glucose to 5‐hydroxymethylfurfural with a bifunctional metal‐organic framework

Glucose conversion to 5‐hydroxymethylfurfural (HMF) generally undergoes catalytic isomerization reaction by Lewis acids followed by the catalytical dehydration to HMF with Brönsted acid. In this work, a sulfonic acid functionalized metal‐organic framework MIL‐101(Cr)‐SO₃H containing both Lewis acid and Brönsted acid sites, was examined as the catalyst for γ‐valerolactone‐mediated cascade reaction of glucose dehydration into HMF. Under the optimal reaction conditions, the batch heterogeneous reaction gave a HMF yield of 44.9% and selectivity of 45.8%. Reaction kinetics suggested that the glucose isomerization in GVL with 10 wt % water follows the second‐order kinetics with an apparent activation energy of 100.9 kJ mol^?1. Continuous reaction in the fixed‐bed reactor showed that the catalyst is highly stable and able to provide a steady HMF yield. This work presents a sustainable and green process for catalytic dehydration of biomass‐derived carbohydrate to HMF with a bifunctional metal‐organic framework. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4403–4417, 2016     

A highly stable metal‐organic framework with optimum aperture size for CO2 capture

We herein report an optimal modulated hydrothermal (MHT) synthesis of a highly stable zirconium metal‐organic framework (MOF) with an optimum aperture size of 3.93 Å that is favorable for CO₂ adsorption. It exhibits excellent CO₂ uptake capacities of 2.50 and 5.63 mmol g^?1 under 0.15 and 1 bar at 298 K, respectively, which are among the highest of all the pristine water‐stable MOFs reported so far. In addition, we have designed a lab‐scale breakthrough set‐up to study its CO₂ capture performance under both dry and wet conditions. The velocity at the exit of breakthrough column for mass balance accuracy is carefully measured using argon with a fixed flow rate as the internal reference. Other factors that may affect the breakthrough dynamics, such as pressure drop and its impact on the roll‐up of the weaker component have been studied in details. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4103–4114, 2017     

经典密度泛函理论在双电层超级电容器研究中的应用

提高储能密度是目前超级电容器研究的重点,它取决于电极材料与电解液界面结构。本文介绍了经典密度泛函理论（CDFT）研究固液界面结构的基本原理以及在多孔电极材料中电解液溶液的热力学和动力学性质研究等进展。CDFT是一种基于统计力学的理论方法,被广泛应用于表界面效应、吸附、溶解等研究,在保证相同计算精度的前提下,具有比分子模拟更高的计算效率。CDFT可以系统地研究多孔材料孔径、孔几何形貌、表面官能团,电解液离子大小、化合价、组成以及溶剂种类、浓度等因素对超级电容器性能的影响,进一步发展考虑反应-传递性质的CDFT,可以为设计新型电极材料和筛选电解液提供理论依据。     

Simulation on hydrogen storage properties of metal‐organic frameworks Cu‐BTC at 77–298 K

In recent years, many researchers have studied on the hydrogen storage properties of metal‐organic frameworks (MOFs) by grand canonical Monte Carlo (GCMC) simulation. At present, the GCMC studies of Cu‐BTC (BTC: benzene‐1,3,5‐tricarboxylate) which is a prototypical metal‐organic framework mainly adopt the classical force fields, the simulation temperatures are mainly focus on 298 and 77 K, and most researchers did not consider the effects of quantum effects at low temperature. Therefore, we used the quantum effects to correct the classical force fields and the force fields with more accurate simulation results were used to simulate the hydrogen adsorption performances of Cu‐BTC in the temperature range of 77–298 K and the pressure range of 1–8 MPa at each temperature. The results show that the effects of quantum effects on the hydrogen storage of Cu‐BTC cannot be neglected and the corrected Dreiding force field can simulate hydrogen adsorption performances of Cu‐BTC more accurately at low temperature. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1383–1388, 2018     

Advanced Monte Carlo simulations of the adsorption of chiral alcohols in a homochiral metal‐organic framework

Grand canonical Monte Carlo (GCMC) simulations with configurational biasing were used to study the enantioselective adsorption of four alkanols in a homochiral metal‐organic framework, known as hybrid organic‐inorganic zeolite analogue HOIZA‐1. Conventional GCMC simulations are not able to converge satisfactorily for this system due to the tight fit of the chiral alcohols in the narrow pores. However, parallel tempering and parallel mole‐fraction GCMC simulations overcome this problem. The simulations show that the enantioselective adsorption of the different (R,S)‐alkanols is due to the specific geometry of the chiral molecules relative to the pore size and shape. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2324–2334, 2014     

Amidinothiourea-linked covalent organic framework for the adsorption of heavy metal ions

For the first time, a novel amidinothiourea-linked covalent organic framework (COF) (TpAt) has been successfully synthesized by the condensation between 1,3,5-triformylphloroglucinol and amidinothiourea via vacuum solvothermal reaction, and was further utilized for the adsorption of metal ions from aqueous solution. The effects of initial concentration, pH and contact time on the adsorption process were investigated. As a result, the TpAt COF showed maximum binding capacities as high as 95.44, 100.76 and 99.08 mg g^–1 for Cr(III), Cd(II) and Cu(II) within 180 min at the initial concentration of 100 mg L^–1, respectively. The isotherms of metal ions onto TpAt could be described by the Freundlich isotherm equation, and the adsorption kinetics followed the pseudo-second-order model. The TpAt could be reused for more than five adsorption–elution cycles, whilst basically maintaining its original adsorption performance and the structural integrity of the COF layers. The robust adsorption efficiency can be attributed to the coordination between metal ions and N, O and S atoms in the TpAt framework. The TpAt COF represents an ideal candidate for the removal of heavy metal ions in environmental pollution treatment. © 2021 Society of Industrial Chemistry.     

Heterometallic cluster-based organic frameworks as highly active electrocatalysts for oxygen reduction and oxygen evolution reaction: a density functional theory study

Recently, metal–organic frameworks are one of the potential catalytic materials for electrocatalytic applications. The oxygen reduction reaction and oxygen evolution reaction catalytic activities of heterometallic cluster-based organic frameworks are investigated using density functional theory. Firstly, the catalytic activities of heterometallic clusters are investigated. Among all heterometallic clusters, Fe₂Mn–Mn has a minimum overpotential of 0.35 V for oxygen reduction reaction, and Fe₂Co–Co possesses the smallest overpotential of 0.32 V for oxygen evolution reaction, respectively 100 and 50 mV lower than those of Pt(111) and RuO₂(110) catalysts. The analysis of the potential gap of Fe₂M clusters indicates that Fe₂Mn, Fe₂Co, and Fe₂Ni clusters possess good bifunctional catalytic activity. Additionally, the catalytic activity of Fe₂Mn and Fe₂Co connected through 3,3′,5,5′-azobenzenetetracarboxylate linker to form Fe₂M–PCN–Fe₂M is explored. Compared with Fe₂Mn–PCN–Fe₂Mn, Fe₂Co–PCN–Fe₂Co, and isolated Fe₂M clusters, the mixed-metal Fe₂Co–PCN–Fe₂Mn possesses excellent bifunctional catalytic activity, and the values of potential gap on the Mn and Co sites of Fe₂Co–PCN–Fe₂Mn are 0.69 and 0.70 V, respectively. Furthermore, the analysis of the electron structure indicates that constructing a mixed-metal cluster can efficiently enhance the electronic properties of the catalyst. In conclusion, the mixed-metal cluster strategy provides a new approach to further design and synthesize high-efficiency bifunctional electrocatalysts.     

介孔金属有机框架材料吸附水中重金属离子研究进展

介孔金属有机框架材料（介孔MOFs）相较于传统吸附剂具有孔径大、孔隙率可调、比表面积大、官能团丰富,便于功能化改性修饰等优点,可高效地吸附水体中重金属污染物。本文介绍介孔MOFs的特性、合成策略及四种合成介孔MOFs的方法,重点分析四种方法的介孔形成机理及其所面临的问题,并将四种合成方法的优劣进行了比较。详述介孔MOFs吸附去除水中重金属离子、类重金属阴离子以及放射性金属离子的研究进展;介绍了介孔MOFs在吸附去除重金属离子方面的可重复利用性;阐述介孔MOFs吸附去除水中重金属污染物的作用机理。对介孔MOFs成本高昂、合成条件苛刻、回收利用难等问题提出了优化方向,指出提高介孔MOFs的水稳定性、易回收利用、简便绿色合成技术以及痕量去除将是未来的研究方向。     

A cobalt metal‐organic framework with small pore size for adsorptive separation of CO2 over N2 and CH4

In this study, a new cobalt‐based metal‐organic framework (MOF), [ (μ₃‐OH)₂(ipa)₅(C₃O₂)(DMF)₂] (CoIPA) was synthesized. The crystal structure analysis shows that CoIPA is constructed by Co₆(μ₃‐OH)₂ units linked by isophthalic acid forming a sxb topology and it possesses a small pore size of about 4 Å. The new MOF has been characterized using multiple experimental methods. Monte Carlo and Molecular Dynamic simulations were employed to investigate adsorption equilibrium and kinetics in terms of capacity and diffusivity of CO₂, N₂, and CH₄ on CoIPA. The gas adsorption isotherms collected experimentally were used to verify the simulation results. The activated CoIPA sample exhibits great gas separation ability at ambient conditions for CO₂/N₂ and CO₂/CH₄ with selectivity of around 61.4 and 11.7, respectively. The calculated self‐diffusion coefficients show a strong direction dependent diffusion behavior of target molecules. This high adsorption selectivity for both CO₂/N₂ and CO₂/CH₄ makes CoIPA a potential candidate for adsorptive CO₂ separation. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4532–4540, 2017     

Metal‐organic frameworks for highly efficient adsorption of dibenzothiophene from liquid fuels

By taking desulfurization of liquid fuels as a demonstrative example, a bottom‐up selection was performed to find the metal‐organic frameworks (MOF)‐type adsorbents with highly efficient adsorption performance of large molecules. Through carefully analyzing the adsorption mechanism for typical S‐heterocyclic compounds like dibenzothiophene (DBT), PCN‐10 was selected in consideration of the simultaneous inclusion of several kinds of interactions in the framework. Experimental results demonstrate that this MOF exhibits extraordinary high DBT adsorption capacity (75.24 mg S g^?1), showing record uptake among all the reported porous materials for the removal of thiophenicsulfur from fuels (below 1000 ppm_wS), to the best of our knowledge. Moreover, the removal rate for the low sulfur concentration (50 ppm_wS) can reach beyond 99%. This strategy can be conveniently extended to the screening and design of MOFs for the efficient removal of other important large guest molecules. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4491–4496, 2016     

Functionalized metal‐organic polyhedra hybrid membranes for aromatic hydrocarbons recovery

Pervaporation membranes are potentially useful in the separation of aromatic/aliphatic mixtures. Wherein, the membrane material plays a key role. Herein, a series of functionalized metal‐organic polyhedra (MOPs)/hyperbranched polymer hybrid membranes are molecularly designed and fabricated for the recovery of aromatic hydrocarbons. The isostructural MOP molecules with different functional groups are uniform in shape/size and soluble in solvents, which enable them to disperse well and be compatible in/with the polymer. Pervaporation results demonstrated significant improvements of these membranes in separation performances. Particularly, the membrane with MOP‐SO₃Na_nH_m showed the separation factor of 8.03 and the permeation flux of 528 g/m²h for the recovery of toluene from its 50 wt % n‐heptane mixture, and those values are 8.4 and 540 g/m²h for benzene/cyclohexane mixture. We propose that the selectivity of these membranes is affected primarily by the polarity of functional groups in MOPs, which were further explained by the adsorption experiments and molecular simulations. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3706–3716, 2016